It is an M Plan APO type. Plan means "Planar", therefore it can cover a bigger area of an image sensor - (the last photo was taken with Canon EOS 5D mk2, which is a full-frame camera). APO means "APOchromatic", which is the highest grade of optical corrections in a microscope objective, therefore less colour fringing (less colour fringing ONLY in the middle of the image, extreme sides can show some colour fringing). Of course, please don't take my word for it.
210/0 means that it requires to be 210mm from the eyepiece or from the camera sensor (so you would need some hefty stack of extension tubes). These types of objectives are NOT built to cover the 36x24mm image sensor of your full-frame camera in full sharpness from edge to edge. 0.90 is a numerical aperture of an objective which means resolution, which is pretty high for a 50x lens. Therefore the working distance is extremely small. I think it is smaller than 1 millimetre.
It could be 0.5mm. I believe this lens would only benefit someone who can use it for glass microscope slides of some little microorganisms or something similar to it, where the object is sliced in between the microscope glass. It is not very much usable for ordinary microscope photography as the working distance is just unbearable for fitting any light in between the lens and the object, therefore it needs to be lightened from behind the photographed object that's thin enough to let the light into the objective. The objective is "pushable" in order not to damage a microscope slide, so if you push it inside, you just need to shake it a bit, and it will come back to a normal position. At least, that's how it worked for me.
I wasn't able to test it very properly because of lack of time and microscope slides that I don't have, but the objective looks good to me, and I was able to do a couple of pictures of an ordinary paper with some atrament writing on it (which is the last picture). M Plan objectives are metallurgical; 210 tube length adds 50mm for vertical illuminator space to frame 160 mm tube length. Illumination is intended to shine down thru the objective and reflect back thru the beam splitter to the head.
This works best when illumination is polarized at an angle different from the analyzer. "210/0" designation indicates that no cover slip is wanted. 0.90 NA probably results in a working distance less than the cover slip thickness.
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